Critical to the use of retroviral-mediated gene transfer for protocols of somatic cell human gene therapy is efficient and safe gene transfer in vitro. Efficiency of retrovirus mediated transduction is determined by several factors including the quality of virus preparation as well as properties of the recipient cell. The abundance of unoccupied virus receptor on the surface of cells may limit the efficiency of gene transfer. All recombinant retroviruses currently available for human gene transfer are based on an amphotropic strain that is internalized by a specific, poorly characterized, cell surface receptor. The overall goal of this project is to create recombinant retroviruses capable of substantially improved gene transfer into human cells. Phase I studies will attempt to generate recombinant retroviruses based on a xenotropic strain of virus which is internalized via a receptor distinct from that used by amphotropic virus. The efficiency of transduction of human cells by the xenotropic recombinant retrovirus will be compared to results with amphotropic based virus. Phase II studies will attempt to further improve the efficiency of retrovirus mediated gene transfer by creating virus packaging cell lines that overexpress the protein components necessary for virion formation.